1. Field of the Invention
The present invention concerns an antenna arrangement for a magnetic resonance apparatus of the type having at least four individually operable antenna conductor loops arranged like a matrix in rows and columns, wherein two antenna conductor loops adjacent in a row or in a column are inductively decoupled from one another. Moreover, the invention concerns a method to measure magnetic resonance signals with such an antenna arrangement.
2. Description of the Prior Art
In a magnetic resonance examination of specific organs or body parts of a patient, antennas known as surface antennas are used to acquire the magnetic resonance signals (MR signals). In the examination these surface antennas are arranged relatively close to the organ or body part to be examined, advantageously directly on the body surface of the patient. In contrast to antennas arranged remote from the patient that are normally used to generate an entire slice image through a patient, these surface antennas consequently have the advantage that they are arranged closer to the regions of interest, and therefore have a greater fill factor. The usable signal is thereby less attenuated and the component portion caused by electrical losses within the body of the patient is thereby reduced, meaning that the signal-to-noise ratio (SNR) of a surface antenna is in principle better than that of a more remote antenna. However, it is disadvantageous that a single surface antenna is able to generate an effective image only within a specific spatial expanse that lies in the range of the diameter of the conductor loop of the surface antenna. The usage possibilities for such individual surface antennas therefore are very limited due to the limited observation region (field of view). The observation region can be expanded by enlarging the diameter of the conductor loop of the surface antenna, but an increase of the electrical losses in the body of the patient, and therefore attendant greater noise, is associated with the enlargement of the conductor loop. With the use of a single surface antenna, a middle path must therefore be chosen between optimally good resolution and an optimally large observation region. One possibility to enlarge the observation region without reducing the resolution to the same degree is to use a field of smaller surface antennas arranged adjacent to one another, for example the aforementioned antenna arrangement with multiple individually operable antenna conductor loops arranged like a matrix in rows and columns, which antenna conductor loops form a large surface antenna.
One problem in the use of such an antenna arrangement with multiple adjacent antenna conductor loops is that a radio-frequency current in one antenna conductor loop can induce a voltage in an adjacent antenna conductor loop. This is typically referred to inductive coupling of the antenna conductor loops. The inductive coupling leads to the situation that a signal generated in one of the adjacent conductor loops automatically also causes a signal component in the adjacent antenna conductor loop. The inductive coupling consequently impairs the signal-to-noise ratio. The outlay for evaluation of the signals of coupled antenna conductor loops is larger in the case of uncoupled antenna conductor loops. Inductive coupling of the antenna conductor loops therefore should optimally be avoided.
One possibility to decouple adjacent antenna conductor loops is to arrange the adjacent conductor loops so as to overlap to a specific degree, so that the inductive coupling between the appertaining antenna conductor loops is minimal overall.
Such a “geometric” or “inductive” decoupling is possible without further measures for antenna conductor loops that lie immediately adjacent to one another within a row or column of the antenna arrangement. The decoupling of conductor loops that adjoin one another diagonally in adjacent rows and columns is problematic. In order to achieve geometric decoupling between these conductor loops, the geometry of the conductor loops would have to be significantly altered, causing the antenna characteristic to be significantly affected. An overlap of the diagonally adjacent coils would also lead to a larger central region being simultaneously covered by four coils. This leads to an unwanted signal increase in this region, which among other things can lead to image artifacts.
In practice, surface antennas are therefore generally used in which the conductor loops diagonally adjacent to one another are not decoupled at all within the antenna arrangement, but rather in which it is sought to achieve as good a decoupling as possible merely by a sufficient distance between the conductors of the diagonally adjacent antenna conductor loops and by suitable decoupling of the preamplifiers arranged at the antenna terminals.
Such an arrangement according to the prior art is shown in FIG. 1. Therein, four conductor loops L1, L2, L3, L4 are arranged like a matrix relative to one another redundancy information an antenna arrangement. The signals acquired by means of the antenna conductor loops L1, L2, L3, L4 are respectively tapped via the terminals A and transmitted via preamplifiers to the components processing the signals further. An inductive decoupling via a suitable overlap U of the respective antenna conductor loops L1, L2, L3, L4 is achieved between the respective conductor loops L1, L2, L3, L4 directly adjacent a row or column. In contrast to this, a greater separation is maintained between two respective antenna conductor loops L1, L2, L3, L4 diagonal to one another. As FIG. 1 shows, this leads to the situation that the central region Z is not covered by the antenna conductor loops L1, L2 L3, L4, which is likewise not optimal.
In order to achieve a decoupling of diagonal elements, in some antenna arrangements external conductor structures shaped like butterfly wings and lying at a free potential are used, with these conductor structures being dimensioned, and attached in the diagonals by the diagonally adjacent antenna conductor loops, so that the magnetic flux that couples the conductor loops situated diagonally relative to one another is increased again. This leads to the situation, however, of an increased copper requirement exists for the additional conductor loops, which in turn reduces the quality factor of the entire antenna arrangement and the mechanical resilience, in particular for flexible coils.